Aulacaspis yasumatsui Takagi invaded Guam in 2003 (Marler and Muniappan, 2006). At that time, Cycas micronesica (Fig. 1A) was the most dominant tree species in natural forests (Donnegan et al., 2004). Furthermore, this native cycad species and the internationally popular Cycas revoluta Thunb. were dominant in the residential and commercial landscape at the time (Marler, 2012). The armored scale killed landscape trees by the end of 2005 unless chemical protection was employed. Furthermore, the pest was largely responsible for killing more than 90% of the C. micronesica plants in Guam’s forests (Marler and Lawrence, 2012).
Loss of host species following invasions of specialist phytophagous insects is occurring with greater frequency. Meaningful studies are needed to evaluate how the systems change following those losses, a need that is heightened when the host species is a dominant species (Marler and Lawrence, 2013). When these same insects become horticultural pests, increases in costs for pest protection and decreases in aesthetic value of the host species may occur. Therefore, some invasive insect species can eliminate the appeal of growing their host species in home gardens, commercial orchards, or agroforestry settings.
The initial pulse of scale-infested C. micronesica leaf litter was substantial shortly after the scale immigrated into new habitats (Fig. 1B). Thereafter, continuing inputs of dead leaf and stem tissue to the forest floor opened up canopy gaps as the plants were killed (Fig. 1C). Root and stem decomposition then increased biochemical inputs to the soil matrix from dead cycad tissue. Unlike other canopy gaps in Guam, the open soil surfaces often failed to recruit new plants from the seed bank or immigration (Fig. 1C). These barren sites had received no pesticide applications, so observations indicated a phytotoxic chemical legacy may be causal. However, no effort has been made to understand this phenomenon. If scale-infested cycad litter is validated as a source of phytotoxic compounds, horticulturists would become more informed for making management decisions in areas infested with A. yasumatsui.
The objective of our study was to examine if a chemical legacy effect is linked with the lack of seedling growth in soils with a history of litter from recently killed C. micronesica trees. We used standard allelopathy-based research protocols using activated charcoal as a substance that adsorbs organic compounds in combination with fertilization to correct for any influence of the activated charcoal on soil nutrient availability (Inderjit and Callaway, 2003; Scharfy et al., 2011). We selected papaya (Carica papaya) and bitter melon (Momordica charantia) for the bioassays for two reasons. First, these two naturalized pioneer species are among the most rapid to emerge whenever canopy gaps occur in Guam’s limestone forests. Second, improved genotypes of these two species are grown in home gardens and commercial farms in Guam and all other Micronesian islands. Our results should inform management decisions when cycads are used in horticultural and agroforestry settings and A. yasumatsui infestations are problematic.
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